Centromeres. Methods of division of somatic and germ cells Divergence of chromosomes in meiosis

They are double-stranded, replicated chromosomes that are formed during division. The main function of the centromere is to serve as an attachment site for spindle fibers. The spindle elongates cells and separates chromosomes to ensure that each new one receives the correct number of chromosomes when completed or.

The DNA in the centromeric region of the chromosome is composed of tightly packed DNA, known as heterochromatin, which is highly compacted and therefore not transcribed. Due to the presence of heterochromatin, the centromere region is stained with dyes darker than other parts of the chromosome.

Location

The centromere is not always located in the central region of the chromosome (see photo above). A chromosome consists of a short arm (p) and a long arm (q), which join at the centromere region. Centromeres can be located either near the middle or in several positions along the chromosome. Metacentric centromeres are located near the center of the chromosomes. Submetacentric centromeres are shifted to one side from the center, so that one arm is longer than the other. Acrocentric centromeres are located near the end of the chromosome, and telocentric centromeres are located at the end or in the telomere region of the chromosome.

The position of the centromere is easily detected in the human karyotype. Chromosome 1 is an example of a metacentric centromere, chromosome 5 is an example of a submetacentric centromere, and chromosome 13 is an example of an acrocentric centromere.

Chromosome segregation in mitosis

Before mitosis begins, the cell enters a stage known as interphase, where it replicates its DNA in preparation for cell division. Sisters are formed, which are connected at their centromeres.

During prophase of mitosis, specialized areas on the centromeres called kinetochores attach chromosomes to spindle fibers. Kinetochores are composed of a series of protein complexes that generate kinetochore fibers that attach to the spindle. These fibers help manipulate and separate chromosomes during cell division.

At the metaphase stage, chromosomes are held on the metaphase plate by equal forces of polar fibers, pressing on the centromeres.

During anaphase, paired centromeres on each individual chromosome begin to diverge from each other as they first center themselves relative to the opposite poles of the cell.

During telophase, the newly formed ones include individual daughter chromosomes. After cytokinesis, two different ones are formed.

Chromosome segregation in meiosis

In meiosis, the cell goes through two stages of the division process (meiosis I and meiosis II). During metaphase I, the centromeres of homologous chromosomes are oriented to opposite poles of the cells. This means that homologous chromosomes will attach at their centromeric regions to spindle fibers extending from only one of the two poles of the cell.

When spindle fibers contract during anaphase I, homologous chromosomes are pulled toward opposite poles of the cells, but sister chromatids remain together. In meiosis II, spindle fibers extending from both cell poles attach to sister chromatids at their centromeres. Sister chromatids separate in anaphase II, when spindle fibers pull them toward opposite poles. Meiosis results in the separation and distribution of chromosomes among four new daughter cells. Each cell contains only half the number of chromosomes of the original cell.

Depending on the functional and physiological states, a cell can divide in different ways. Division methods somatic cells: mitosis, amitosis or endomitosis. Sex cells divide by meiosis.

Mitosis – indirect cell division, accompanied by spiralization of chromosomes. There are several phases in mitosis:

I Prophase (from the Greek “pro” - before, “phases” - appearance). Spiralization and shortening of chromosomes occurs. The nucleolus and nuclear envelope disappear, the centrioles diverge to the poles of the cell, and a spindle is formed. Chromosomes consist of two chromatids connected by a centromere. Prophase is the longest phase of mitosis. Set of genetic material – 2n 4c.

II Metaphase (from the Greek “meta” - middle). Chromosomes, consisting of two chromatids, line up in the equatorial plane of the cell. The spindle filaments are attached to the centromeres. There are two types of filaments in the division spindle: 1) chromosomal, associated with the primary constrictions of chromosomes, 2) centrosomal, connecting the division poles. The set of genetic material at this moment is 2n 4c.

III Anaphase (from the Greek “ana” - up). The shortest division phase. The centromeres of the chromosomes are separated, and the chromatids (daughter chromosomes) become independent. The spindle filaments attached to the centromeres pull the daughter chromosomes to the poles of the cell. Set of genetic material – 2n 2c.

IV Telophase. Chromosomes, consisting of one chromatid, are located at the poles of the cell. Chromosomes despiral (unwind). At each pole, a nuclear membrane and nucleoli are formed around the chromosomes. The spindle threads disintegrate. The cytoplasm of the cell is divided (cytokinesis = cytotomy). Two daughter cells are formed. The set of genetic material of daughter cells is 2n 2c.

The separation of the cytoplasm by a constriction occurs differently in different cells. In animal cells, the invagination of the cytoplasmic membrane inward during cell division occurs from the edges to the center. In plant cells, a partition is formed in the center, which then increases towards the cell walls.

Biological significance of mitosis. Mitosis results in the precise distribution of genetic material between two daughter cells. Daughter cells receive the same set of chromosomes that the mother cell had - diploid. Mitosis ensures the maintenance of a constant number of chromosomes over a number of generations and serves as a cellular mechanism for growth, development of the body, regeneration and asexual reproduction. Mitosis is the basis for asexual reproduction of organisms. The number of daughter cells formed during mitosis is 2.

Amitosis(from the Greek “a” - negation, “mitos” - thread) - direct cell division, in which the nucleus is in an interphase state. Chromosomes are not detected. Division begins with changes in the nucleoli. Large nucleoli are divided by a constriction. Following this, the nucleus divides. The nucleus can be separated by only one constriction or fragmented. The resulting daughter nuclei may be of unequal size.

That. amitosis leads to the appearance of two cells with nuclei of different sizes and numbers. Often, after amitosis, two cells are not formed, i.e. After nuclear divisions, separation of the cytoplasm (cytokinesis) does not occur. 2 and multinucleate cells are formed. Amitosis occurs in aging, degenerating somatic cells.

Endomitosis- a process in which the doubling of chromosomes in a cell is not accompanied by nuclear division. As a result, the number of chromosomes in the cell multiplies, sometimes tens of times compared to the original number. Endomitosis occurs in intensively functioning cells.

Sometimes reproduction of chromosomes occurs without increasing their number in the cell. Each chromosome doubles many times, but the daughter chromosomes remain connected to each other (the phenomenon of polyteny). As a result, giant chromosomes are formed.

Meiosis - a special form of cell division in which haploid daughter cells are formed from diploid maternal germ cells. The fusion of male and female haploid gametes during fertilization leads to the appearance of a zygote with a diploid set of chromosomes. As a result, the daughter organism developing from the zygote has the same diploid karyotype that the mother organism had.

Meiosis involves two successive divisions.

The first meiotic division is called reduction. It includes 4 stages.

Prophase I. The longest stage. It is conventionally divided into 5 stages.

1) Leptotene. The core increases. The spiralization of chromosomes begins, each of which consists of two chromatids.

2) Zygotene. Conjugation of homologous chromosomes occurs. Homologous are chromosomes that have the same shape and size. Chromosomes attract and adhere to each other along their entire length.

3) Pachytene. The convergence of chromosomes ends. Double chromosomes are called bivalents. They consist of 4 chromatids. The number of bivalents = the haploid set of chromosomes of the cell. Chromosome spiralization continues. Close contact between chromatids makes it possible to exchange identical regions in homologous chromosomes. This phenomenon is called crossing over (crossing of chromosomes).

4) Diplotene. Chromosome repulsive forces arise. The chromosomes that make up the bivalents begin to move away from each other. At the same time, they remain connected to each other at several points - chiasmata. Crossing over may occur in these locations. Further spiralization and shortening of chromosomes occurs.

5) Diakinesis. The repulsion of chromosomes continues, but they remain connected at their ends into bivalents. The nucleolus and nuclear envelope dissolve, the filaments of the spindle diverge towards the poles. Set of genetic material – 2n 4c.

Metaphase I. Chromosome bivalents are located along the equator of the cell, forming a metaphase plate. The spindle threads are attached to them. Set of genetic material – 2n 4c.

Anaphase I. Chromosomes move towards the poles of the cell. Only one of a pair of homologous chromosomes reaches the poles. Set of genetic material – 1n 2c.

Telophase I. The number of chromosomes at each pole of the cell becomes haploid. Chromosomes consist of two chromatids. At each pole, a nuclear envelope is formed around a group of chromosomes, the chromosomes despiral, and the nucleus becomes interphase. Set of genetic material – 1n 2c.

After telophase I, cytokinesis begins in an animal cell, and cell wall formation begins in a plant cell.

Interphase II only found in animal cells. There is no DNA duplication.

Meiotic division II is called equatorial division. It is similar to mitosis. The difference from mitosis is that from chromosomes having two chromatids, chromosomes consisting of one chromatid are formed. Meiotic division II differs from mitosis in that during division two groups of chromosomes and, accordingly, two spindles are formed in the cell. The set of genetic material in prophase II is 1n 2c, starting from metaphase II - 1n 1c.

Biological significance of meiosis. Leads to a halving of the number of chromosomes, which determines the constancy of species on Earth. If the number of chromosomes did not decrease, then in each subsequent generation the chromosomes would double. Provides heterogeneity of gametes in gene composition (crossing over can occur in prophase, free recombination of chromosomes can occur in metaphase). A chance meeting of germ cells (=gametes) – a sperm and an egg with a different set of genes – causes combinative variability. Parents' genes are combined during fertilization, so their children may develop traits that the parents did not have. The number of cells formed is 4.

Functions

The centromere is involved in the connection of sister chromatids, kinetochore formation, conjugation of homologous chromosomes and is involved in the control of gene expression.

It is in the centromere region that sister chromatids are connected in prophase and metaphase of mitosis and homologous chromosomes in prophase and metaphase of the first division of meiosis. At centromeres, kinetochores are formed: proteins that bind to the centromere form an attachment point for spindle microtubules in anaphase and telophase of mitosis and meiosis.

Deviations from the normal functioning of the centromere lead to problems in the relative position of chromosomes in the dividing nucleus, and as a result, to disruptions in the process of chromosome segregation (their distribution between daughter cells). These disorders lead to aneuploidy, which can have severe consequences (for example, Down syndrome in humans associated with aneuploidy (trisomy) on chromosome 21).

Centromeric sequence

In most eukaryotes, the centromere does not have a specific nucleotide sequence corresponding to it. It typically consists of a large number of DNA repeats (eg, satellite DNA) in which the sequence within the individual repeat elements is similar but not identical. In humans, the main repeat sequence is called the α-satellite, but there are several other types of sequences in this region. It has been established, however, that α-satellite repeats are not sufficient for kinetochore formation and that functioning centromeres are known that do not contain α-satellite DNA.

Inheritance

Epigenetic inheritance plays a significant role in determining the location of the centromere in most organisms. Daughter chromosomes form centromeres in the same places as the mother chromosome, regardless of the nature of the sequence located in the centromeric region. It is assumed that there must be some primary way of determining the location of the centromere, even if its location is subsequently determined by epigenetic mechanisms.

Structure

Centromere DNA is usually represented by heterochromatin, which may be essential for its functioning. In this chromatin, normal histone H3 is replaced by the centromere-specific histone CENP-A (CENP-A is characteristic of baker's yeast S. cerevisiae, but similar specialized nucleosomes appear to be present in all eukaryotic cells). The presence of CENP-A is thought to be required for kinetochore assembly at the centromere and may play a role in the epigenetic inheritance of centromere location.

In some cases, for example in the nematode Caenorhabditis elegans, in Lepidoptera, as well as in some plants, chromosomes holocentric. This means that the chromosome does not have a characteristic primary constriction- a specific area to which spindle microtubules are predominantly attached. As a result, kinetochores are diffuse in nature, and microtubules can attach along the entire length of the chromosome.

Centromere aberrations

In some cases, a person has noted the formation of additional neocentromere. This is usually combined with inactivation of the old centromere, since dicentric chromosomes (chromosomes with two active centromeres) are usually destroyed during mitosis.

In some unusual cases, spontaneous formation of neocentromeres on fragments of broken chromosomes has been noted. Some of these new positions were originally composed of euchromatin and did not contain alpha satellite DNA at all.

see also

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See what "Centromere" is in other dictionaries:

centromere- centromere... Spelling dictionary-reference book

centromere- kinetochore Dictionary of Russian synonyms. centromere noun, number of synonyms: 1 kinetochore (1) ASIS Dictionary of Synonyms. V.N. Trishin... Synonym dictionary

CENTROMERE- (from center and Greek meros part) (kinetochore) a section of a chromosome that holds its two strands (chromatids) together. During division, centromeres direct the movement of chromosomes towards the poles of the cell... Big Encyclopedic Dictionary

CENTROMERE- CENTROMERE, part of the CHROMOSOME, which appears only during the process of cell division. When chromosomes contract during MEIOSIS or MITOSIS, centromeres arise as constrictions that do not contain any genes. With their help, chromosomes are attached to... ... Scientific and technical encyclopedic dictionary

CENTROMERE- (from Latin centrum, Greek kentron midpoint, center and Greek meros part, share), kinetochore, a section of a chromosome that controls its movement to different poles of the cell during mitosis or meiosis; place of attachment of threads to the chromosome... ... Biological encyclopedic dictionary

centromere- A limited zone in the chromosome, including the spindle attachment site during mitosis or meiosis. Biotechnology topics EN centromere ... Technical Translator's Guide

Centromere- * centramere * centromere or kinetochore a conserved region of the eukaryotic chromosome to which the spindle threads (see) are attached during mitosis (see). The DNA that forms the DNA consists of three domains (elements) CDE I, CDE II and CDE III. CDE I and... Genetics. encyclopedic Dictionary

centromere- (from center and Greek méros part) (kinetochore), a section of a chromosome that holds its two strands (chromatids) together. During division, centromeres direct the movement of chromosomes towards the poles of the cell. * * * CENTROMETER CENTROMETER (from the center (see DIRECT CONTROL) and ... encyclopedic Dictionary

centromere- centromere centromere. A section of a monocentric chromosome in which sister chromatids are connected to each other and in the area of ​​which spindle threads are attached, ensuring the movement of chromosomes to the poles of division; usually pericentromeric regions... ... Molecular biology and genetics. Dictionary.

centromere- centromera statusas T sritis augalininkystė apibrėžtis Pirminė chromosomos persmauga, prie kurios prisitvirtina achromatinės verpstės siūlai. atitikmenys: engl. centromere; kinetochore rus. kinetochore; centromere... Žemės ūkio augalų selekcijos ir sėklininkystės terminų žodynas

Functions

The centromere is involved in the connection of sister chromatids, kinetochore formation, conjugation of homologous chromosomes and is involved in the control of gene expression.

It is in the centromere region that sister chromatids are connected in prophase and metaphase of mitosis and homologous chromosomes in prophase and metaphase of the first division of meiosis. At centromeres, kinetochores are formed: proteins that bind to the centromere form an attachment point for spindle microtubules in anaphase and telophase of mitosis and meiosis.

Deviations from the normal functioning of the centromere lead to problems in the relative position of chromosomes in the dividing nucleus, and as a result, to disruptions in the process of chromosome segregation (their distribution between daughter cells). These disorders lead to aneuploidy, which can have severe consequences (for example, Down syndrome in humans associated with aneuploidy (trisomy) on chromosome 21).

Centromeric sequence

In most eukaryotes, the centromere does not have a specific nucleotide sequence corresponding to it. It typically consists of a large number of DNA repeats (eg, satellite DNA) in which the sequence within the individual repeat elements is similar but not identical. In humans, the main repeat sequence is called the α-satellite, but there are several other types of sequences in this region. It has been established, however, that α-satellite repeats are not sufficient for kinetochore formation and that functioning centromeres are known that do not contain α-satellite DNA.

Inheritance

Epigenetic inheritance plays a significant role in determining the location of the centromere in most organisms. Daughter chromosomes form centromeres in the same places as the mother chromosome, regardless of the nature of the sequence located in the centromeric region. It is assumed that there must be some primary way of determining the location of the centromere, even if its location is subsequently determined by epigenetic mechanisms.

Structure

Centromere DNA is usually represented by heterochromatin, which may be essential for its functioning. In this chromatin, normal histone H3 is replaced by the centromere-specific histone CENP-A (CENP-A is characteristic of baker's yeast S. cerevisiae, but similar specialized nucleosomes appear to be present in all eukaryotic cells). The presence of CENP-A is thought to be required for kinetochore assembly at the centromere and may play a role in the epigenetic inheritance of centromere location.

In some cases, for example in the nematode Caenorhabditis elegans, in Lepidoptera, as well as in some plants, chromosomes holocentric. This means that the chromosome does not have a characteristic primary constriction- a specific area to which spindle microtubules are predominantly attached. As a result, kinetochores are diffuse in nature, and microtubules can attach along the entire length of the chromosome.

Centromere aberrations

In some cases, a person has noted the formation of additional neocentromere. This is usually combined with inactivation of the old centromere, since dicentric chromosomes (chromosomes with two active centromeres) are usually destroyed during mitosis.

In some unusual cases, spontaneous formation of neocentromeres on fragments of broken chromosomes has been noted. Some of these new positions were originally composed of euchromatin and did not contain alpha satellite DNA at all.

see also

Links

Wikimedia Foundation. 2010.

See what "Centromere" is in other dictionaries:

Centromere... Spelling dictionary-reference book

Kinetochore Dictionary of Russian synonyms. centromere noun, number of synonyms: 1 kinetochore (1) ASIS Dictionary of Synonyms. V.N. Trishin... Synonym dictionary

- (from center and Greek meros part) (kinetochore) a section of a chromosome that holds its two strands (chromatids) together. During division, centromeres direct the movement of chromosomes towards the poles of the cell... Big Encyclopedic Dictionary

CENTROMERE, part of a CHROMOSOME that appears only during cell division. When chromosomes contract during MEIOSIS or MITOSIS, centromeres arise as constrictions that do not contain any genes. With their help, chromosomes are attached to... ... Scientific and technical encyclopedic dictionary

- (from Latin centrum, Greek kentron midpoint, center and Greek meros part, share), kinetochore, a section of a chromosome that controls its movement to different poles of the cell during mitosis or meiosis; place of attachment of threads to the chromosome... ... Biological encyclopedic dictionary

centromere- A limited zone in the chromosome, including the spindle attachment site during mitosis or meiosis. Biotechnology topics EN centromere ... Technical Translator's Guide

Centromere- * centramere * centromere or kinetochore a conserved region of the eukaryotic chromosome to which the spindle threads (see) are attached during mitosis (see). The DNA that forms the DNA consists of three domains (elements) CDE I, CDE II and CDE III. CDE I and... Genetics. encyclopedic Dictionary

- (from center and Greek méros part) (kinetochore), a section of a chromosome that holds its two strands (chromatids) together. During division, centromeres direct the movement of chromosomes towards the poles of the cell. * * * CENTROMETER CENTROMETER (from the center (see DIRECT CONTROL) and ... encyclopedic Dictionary

Centromere centromere. A section of a monocentric chromosome in which sister chromatids are connected to each other and in the area of ​​which spindle threads are attached, ensuring the movement of chromosomes to the poles of division; usually pericentromeric regions... ... Molecular biology and genetics. Dictionary.

centromere- centromera statusas T sritis augalininkystė apibrėžtis Pirminė chromosomos persmauga, prie kurios prisitvirtina achromatinės verpstės siūlai. atitikmenys: engl. centromere; kinetochore rus. kinetochore; centromere... Žemės ūkio augalų selekcijos ir sėklininkystės terminų žodynas

No. 9, 2007

© Vershinin A.V.

Centromeres and telomeres of chromosomes

A.V. Vershinin

Alexander Vasilievich Vershinin, Doctor of Biological Sciences, Chief Scientific Associate. Institute of Cytology and Genetics SB RAS.

Today almost everyone knows what chromosomes are. These nuclear organelles, in which all genes are localized, constitute the karyotype of a given species. Under a microscope, chromosomes look like uniform, elongated dark rod-shaped structures, and the picture you see is unlikely to seem an intriguing sight. Moreover, preparations of chromosomes of a great many living creatures living on Earth differ only in the number of these rods and modifications of their shape. However, there are two properties that are common to chromosomes of all species. The first is the presence of obligatory compression (or constriction), located either in the middle, or shifted to one of the ends of the chromosome, called the “centromere”. The second is the presence at each end of the chromosome of a specialized structure - telomeres (Fig. 1). Various genes located along the arms (parts of the chromosome from the centromere to the physical end) of the chromosomes, together with DNA regulatory sequences, are responsible for performing a variety of functions. This ensures the uniqueness of the genetic information encoded in each arm of each individual chromosome.

Centromeric and telomeric regions occupy a special position because they perform extremely important, but the same functions in the chromosomes of all types of eukaryotes. Numerous studies have not yet given a clear answer to the question of which molecular structures are responsible for performing these functions and how they carry them out, but obvious progress in this direction has been achieved in recent years.

Before the molecular structure of centromeres and telomeres was elucidated, it was believed that their functions should be determined (encoded) by universal and at the same time region-specific DNA sequences. But direct determination of the primary nucleotide sequence (DNA sequencing) was complicated by the fact that these regions, as a rule, are adjacent to areas of high concentrations of repeating DNA sequences in the chromosomes. What is known today about these functionally important regions of chromosomes?

Centromeres

By the middle of the last century, numerous cytological studies showed the decisive role of the centromere in the morphology of chromosomes. It was later discovered that the centromere, together with the kinetochore (a structure consisting mainly of proteins), is responsible for the correct segregation of chromosomes into daughter cells during cell division. The guiding role of the centromere in this process is obvious: after all, it is to it that the division spindle is attached, which, together with the cell centers (poles), constitutes the cell division apparatus. Due to the contraction of the spindle strands, chromosomes move toward the cell poles during division.

Five stages of cell division (mitosis) are usually described. For simplicity, we will focus on three main stages in the behavior of the chromosomes of a dividing cell (Fig. 2). At the first stage, gradual linear compression and thickening of chromosomes occurs, then a cell division spindle consisting of microtubules is formed. In the second, the chromosomes gradually move toward the center of the nucleus and line up along the equator, probably to facilitate the attachment of microtubules to the centromeres. In this case, the nuclear membrane disappears. At the last stage, the halves of the chromosomes - chromatids - separate. It seems that microtubules attached to the centromeres, like a tugboat, pull the chromatids towards the poles of the cell. From the moment of divergence, the former sister chromatids are called daughter chromosomes. They reach the spindle poles and come together in a parallel pattern. The nuclear envelope is formed.

Rice. 2. Main stages of mitosis.
From left to right: chromosome compaction, spindle formation; alignment of chromosomes along the equator of the cell,
attachment of the spindle to the centromeres; movement of chromatids to the poles of the cell.

With careful observation, one can notice that during the process of cell division in each chromosome, the centromere is in a constant position. It maintains a close dynamic connection with the cell center (pole). Centromere division occurs simultaneously in all chromosomes.

Sequencing methods developed in recent years have made it possible to determine the primary DNA structure of extended sections of human and fruit fly centromeres Drosophila and plants Arabidopsis. It turned out that in the chromosomes of both humans and plants, centromeric activity is associated with a block of tandemly organized DNA repeats (monomers) that are similar in size (170-180 nucleotide pairs, bp). Such sections are called satellite DNA. In many species, including those that are evolutionarily distant from each other, the size of the monomers is almost the same: various species of monkeys - 171 np, corn - 180 np, rice - 168 np, chironomus insect - 155 np. This may reflect general requirements for centromeric function.

Despite the fact that the tertiary structure of human and Arabidopsis centromeres is organized similarly, the primary nucleotide sequences (or nucleotide order) in their monomers turned out to be completely different (Fig. 3). This is surprising for a region of the chromosome that performs such an important and universal function. However, when analyzing the molecular organization of centromeres in Drosophila, a certain structural pattern was discovered, namely the presence of sections of monomers of approximately the same size. Thus, in Drosophila, the centromere of the X chromosome consists mainly of two types of very short simple repeats (AATAT and AAGAG), interrupted by retrotransposons (mobile DNA elements) and “islands” of more complex DNA. All these elements were found in the Drosophila genome and outside the centromeres, but DNA sequences characteristic of each centromere were not found in them. This means that centromeric DNA sequences themselves are insufficient and unnecessary for the formation of a centromere.

Rice. 3. DNA structure in human and plant centromeres.

The rectangles correspond to tandemly organized monomers with identical nucleotide sequences inside (primary DNA structure). In different species, the primary structure of DNA monomers varies, and the secondary structure is a helix. The sequence of monomers reflects the higher level structural organization of DNA.

The functional role of the centromeric chromatin structure is confirmed by the presence of histone H3 variants specific for each biological species in centromeric chromatin: in humans they are called CENP-A, in plants - CENH3. Among the many proteins present in the kinetochore, only two, CENH3 and centromeric protein C (CENP-C), directly bind to DNA. Perhaps it is CENH3, interacting with other histones (H2A, H2B and H4), that forms and determines the type of nucleosomes specific to centromeres. Such nucleosomes can serve as a kind of anchors for kinetochore formation. Variants of histone H3 in centromeres of various species are similar to the canonical histone H3 molecule in areas of interaction with other histone proteins (H2A, H2B, H4). However, the region of centromeric histone H3 that interacts with the DNA molecule appears to be under the influence of driving selection. As discussed, the primary structure of centromeric DNA differs between species, and centromeric histone H3 has been proposed to coevolve with centromeric DNA, particularly in Drosophila and Arabidopsis.

The discovery of the centromeric histone H3 gave rise to the extreme view that centromeric function and its complete independence from the primary DNA structure are determined by the nucleosomal organization and this histone. But are these factors sufficient for full centromere activity? Models that ignore the role of primary DNA structure must assume a random distribution of changes in centromeric DNA structure across populations in the absence of selection. However, analysis of satellite DNA in human centromeres and Arabidopsis identified conserved regions as well as regions with higher than average variability, indicating selection pressure on centromeric DNA. In addition, artificial centromeres were obtained only with human a-satellite repeats amplified from natural centromeres, but not from a-satellites of pericentromeric chromosome regions.

Models in which the decisive factor in determining the position of the centromere (preserved from generation to generation) and its functions is the tertiary (or even higher order) structure of DNA are encountered with fewer fundamental difficulties for explanation. Its conservatism allows for large variations in the nucleotide sequence and does not exclude fine tuning of the primary structure.

In recent years, it has become obvious that there are no universal DNA sequences that directly determine the functions of centromeres and telomeres. In these regions of the chromosomes, DNA serves as a platform for the assembly of complex, multicomponent DNA-protein complexes, which ensure the performance of these functions. More details about the complementary organization of these complexes and their coordinated functioning can be found in our review. Along with centromere- and telomere-specific components of these complexes, they also include those that are involved in performing several functions, sometimes even opposite ones. For example, the Ku70/80 heterodimer is part of telomeres and works as a positive regulator of telomere length in yeast and a negative regulator in Arabidopsis. At the same time, this protein is involved in the recognition of chromosome breaks and their repair. Without a doubt, one of the most relevant areas of research is identifying the molecular nature of the regulatory mechanisms of various molecular complexes that ensure the activity of centromeres and telomeres.

This work was supported by the Russian Foundation for Basic Research (project 04-04-48813), INTAS (03-51-5908)
and the Program of Integration Projects of the SB RAS (project 45/2).

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